Drying the reactor prior to initiating polymerization is an important step in starting a polymerization reactor. In the polymerization of olefins and diolefins, it is generally considered necessary to remove any water present or to reduce it to low levels. In the reactor, water can be present as a physically or chemically adsorbed or absorbed species throughout the system, and it can also exist as free water in dead areas of the reactor system such as in unmixed or unheated flanges and nozzles.
Many methods have been applied to reactor systems with and/or without a seed bed present to minimize the amount of water present in the reactor system. These can include pressure purging one or more times with an inert gas such as nitrogen at an elevated temperature, or flow purging the reactor system with nitrogen or other suitable inert gases at a reduced pressure. A vacuum can be placed on the reactor system, preferably at elevated temperature, to reduce the level of water. The vacuum can be applied repeatedly if desired.
Typical vacuum drying can use pressures from just below atmospheric to about 50 mm Hg, and preferably to about 5 to 10 mm Hg. Such high vacuums are difficult but not impossible to achieve for large scale industrial equipment. Vacuums in the range of about 100 to 750 mm Hg aid in the removal of liquid water from the reaction system. The higher vacuums are required to remove adsorbed water from the reactor surfaces. The reactor may first be heated under pressure and then vented to near atmospheric pressure and the vacuum applied while the reactor is still hot in order to improve the drying step. Ideal temperatures are in the range of about 40 to 130.degree. C. The drying is improved as the time duration of the vacuum increases and is typically in the range of about 5 minutes to 2 hours. Insulating the reactor or the external application of heat to the reactor vessel aid in maintaining a high temperature as the system is vacuum dried.
Hydrocarbons such as ethane, ethylene, propane, propylene, butane, isobutane, 1-butene, n-pentane, isopentane, n-hexane and 1-hexene have been introduced and circulated in the gaseous state to increase the heat capacity of a circulating medium and thus speed drying.
Alkyl aluminum and other alkyl metal compounds such as trimethyl aluminum, triethyl aluminum and diethyl zinc have been introduced to reactor systems in the presence or absence of a seed bed to passivate the system or to serve as scavengers for poisons before commencing polymerization. Generally, prior to the introduction of the aluminum alkyl compound, the reactor system is dried to reduce water levels to at least about 10 to 150 ppmv (concentration of water in the circulating medium)--usually less than 50 ppmv and sometimes less than 5 ppmv. It is obvious to those skilled in the art that the above procedures can be applied in combination to improve the drying of the reactor. As an example, the reactor may be dried under vacuum prior to the introduction of the aluminum alkyl to reduce the formation of reaction products of the aluminum alkyl and water and thus improve reactor start-up, operability and resistance to sheet formation.
The drying step can be an expensive and time consuming process, particularly if water has been introduced into the system during shutdown. Such would be the case, for example, if the reactor internals were water-blasted to remove polymer deposits. Experience has shown that water may lay in dead areas such as flanges, tubing or adjacent piping connected to the reactor, and be difficult to remove. Although the reactor is heated during the drying steps, such peripheral areas do not reach the same high temperatures as the main body of the reactor and are more difficult to dry. Heating these areas individually can improve drying but this may prove to be a cumbersome and expensive option with irreproducible results. Heating methods include insulation, heat tracing and the application of hot water or steam to the external surfaces. These areas may also constructed to be as free-draining back into the main part of the reaction system as possible in order to prevent pooling. It is not uncommon for the reactor moisture level to be measured to about 50 ppmv water within a few hours after beginning the drying procedure, and then remain there for several hours or even days. This usually indicates the presence of a liquid pool of water collected in a low spot in the reaction system, and has been confirmed by disconnecting the piping at the low spot and blowing out the liquid water. Blowing the water out of the line to the atmosphere improves reactor drying. Flushing the water back into a hotter part of the reactor also improves drying. Flushing may be accomplished with gas or liquids, including the compounds of the present invention.
The presence of water may adversely impact polymerizations which employ metallocene, Ziegler-Natta or chromium-based catalysts by the generation of electrical charges in the reactor and reaction medium, leading to sheeting, polymer agglomeration, and/or fouling of the reaction system. The polymerization may also be retarded by the presence of water, causing the reaction to start slowly and to have decreased catalyst productivity. This is especially true for chromium based catalysts such as those based on chromium oxide and silyl chromate.
In one method of operation, the reactor system is dried and started under conditions of polymerization or using a catalyst system that is less sensitive to catalyst poisoning or reactor operational problems associated with the residual water in the polymerization system. After a period of time, typically about 5 to 40 bed-turn-overs, the water is sufficiently scavenged and the conditions or catalysts more sensitive to water can be used without difficulty. The reactor may be operated in the presence of scavengers such as aluminum alkyl cocatalysts during this time to improve the drying. In one method especially applicable to chromium based catalysts, the reactor is operated or baked-out with polymerization at high temperatures of about 90 to 125.degree. C. (often while producing medium to high density polymers) for several bed-turn-overs until the expected productivity of the catalyst and molecular weight of the polymer is achieved. Then the reactor is transitioned to a lower temperature to produce a product of desired properties.
Drying a reactor that last produced polymer using a metallocene catalyst is more difficult than if the reactor last produced polymer using a Ziegler-Natta or chromium catalyst. This is due to increased compositional homogeneity of the metallocene based polymers, causing them to soften and melt at temperatures that are 10 to 15.degree. C. lower than comparable (molecular weight and density) resins prepared using conventional Ziegler-Natta or chromium catalysts. Thus, metallocene containing resins cannot be heated as hot as Ziegler-Natta or chromium containing resins during drying. Because a 10.degree. C. drop in temperature lowers the vapor pressure of water considerably, the corresponding drying times of a reactor and/or seed bed in metallocene polymerization systems are much longer than those for Ziegler-Natta or chromium based resins.
The fluidized reactor polymer seed bed is commonly dried by circulating an inert fluidizing gas through the bed and around a recirculation system consisting of a blower and a heat exchanger that is used to heat the inert gas. The upper limit to the drying temperature is set by the hottest surface in contact with the polymer. Generally, this is the temperature of the heat exchanger and/or the temperature of the heat transfer medium (e.g. water), and it is maintained at or just below the softening or melting point of the polymer. Drying at temperatures in excess of the softening or melting point of the polymer can melt the polymer on the heat exchanger internal surfaces, recirculation lines, distributor plate or walls of the reaction vessel. Even with no bed in the reactor, drying at excessively high temperature can melt residual polymer that remains in the recirculation lines, heat exchanger or distributor plate, leading to blockages and high pressure drops in the system.
A similar situation to that described above can exist in the preparation of polymers having a low melting point whether produced with a metallocene catalyst or not. Examples of such polymers can include VLDPE, EPR, EPDM, polybutadiene, and polymers of vinyl-substituted aromatic compounds such as styrene. Accordingly, there exists a need to more effectively dry reactor systems to remove water before commencing such polymerizations.